The powder coating method is utilized in many fields such as automobile parts, steel tubes, home appliances, household goods, and the like. The powder coating method is carried out to apply powder mainly to a flat surface of an exterior portion having a large area. In many cases, the powder coating method is used to apply the powder to the entire surface of an object to be coated (hereinafter referred to as work). But there is a case in which the work has a portion to be uncoated on its surface. In that case, it is necessary to mask the surface to be uncoated. As normal masking methods, a masking tape and a cap are widely used. In that case, it is necessary for an operator to manually perform these methods. As a method of automating an operation of masking the surface to be uncoated, the method to be carried out by using a masking jig is proposed, as shown in FIG. 8(a). FIG. 8(a) is a sectional view showing a case in which a work which is rotating or at a standstill is coated with coating paint by using the jig. In a case where a portion of a work 17 is coated with coating paint in a direction shown with arrows, a masking jig 18 is disposed in the vicinity of the work 17. But in the case of powder coating and in particular in the case of electrostatic powder coating, a portion 19 into which powder coating paint penetrates is generated in a gap between the masking jig 18 and the surface of the work 17. Thus the above-described powder coating method has a problem that the powder is liable to stick to the portion to be uncoated.
As a countermeasure for this problem, as shown in FIG. 8(b), the method of preventing the powder coating paint from penetrating into the gap between the work 17 and the masking jig 18 by flowing a compressed gas 20 into the gap is known (patent documents 1 through 3).
This method is capable of preventing the powder coating paint from penetrating into the gap between the work and the masking jig by increasing the flow rate of the compressed air unlike the method shown in FIG. 8(a). But in this method, the flow velocity of the air is high at a boundary between the portion to be coated and the portion to be uncoated. As a result, a portion 21 where the thickness of the coated paint decreases is generated. Thus the above-described powder coating method has a problem that the corrosion resistance of the coating film deteriorates.
It is necessary for a drive shaft which transmits a power from an engine of a car to a drive wheel to respond to an angular displacement and an axial displacement caused by a change in a relative positional relationship between the engine and a wheel. Therefore the drive shaft has a construction in which an engine side (inboard side) is equipped with a sliding type constant velocity universal joint and a drive shaft side (outboard side) is equipped with a fixed type constant velocity universal joint, and both constant velocity universal joints are coupled to each other with a metal intermediate shaft.
Both the sliding type and the fixed type constant velocity universal joint incorporated in the drive shaft have a metal outer joint member constructed of a cup part accommodating inner parts including an inner joint member coupled to the above-described intermediate shaft and a stem part axially and integrally extended from the cup part.
The metal automotive part composed of the outer joint member of the constant velocity universal joint positioned at the inboard side, the outer joint member of the constant velocity universal joint positioned at the outboard side, and the metal intermediate shaft coupling both constant velocity universal joints to each other is formed by molding materials by forging. Thereafter to enhance the strength of the metal automotive part, the surface thereof is cured by heat treatment of quenching. After the quenching finishes, the part is tempered to enhance the toughness thereof and release a portion of a stress generated owing to the quenching so that occurrence of quenching crack is prevented. A resin coating film is formed on the outer surfaces of these parts to improve corrosion resistance thereof.
As a method of forming the resin coating film, there is disclosed a method and an apparatus for tempering and baking the automotive part by high-frequency induction heating at the same time after subjecting the metal automotive part to induction hardening and thereafter applying powder paint to the outer surface thereof (patent document 4). This method is capable of shortening a treatment period of time required to temper the metal automotive part and bake a coating agent and reducing the cost.
In a case where a precision part such as an outer ring of the constant velocity universal joint is coated with the powder paint, its sliding portion, fitting portion, and gear portion are required to have high dimensional accuracy and high degree of cleanliness. Thus a masking method capable of completely preventing the paint from penetrating thereinto is essentially required. In the method of flowing the air directly to the gap between the masking jig and the work, it is difficult to completely prevent the paint from penetrating into the gap. This method has a problem that the corrosion resistance of the coating film deteriorates owing to a decrease in the thickness of the coating film.
In the method of coating the metal automotive part formed by tempering the quenched automotive part such as the constant velocity universal joint and bake-curing the coating powder paint at the same time, the above-described coating method has a problem that the thickness of the coating film fluctuates because convex and concave portions are present on a paint-applied portion according to the configuration of the inside of an outer ring of the constant velocity universal joint.